The present invention relates to a new process for the manufacture of urea-formaldehyde resins containing melamine; its subject matter is more particularly a process for the manufacture of melamine "doped" urea-formaldehyde resins which give off very little formaldehyde.
Urea-formaldehyde resins are known products which are widely employed in the woodworking industry, particularly for the manufacture of particle boards. They are manufactured in a known manner by condensation of urea and of formaldehyde at a pH of between 4 and 7 and at a temperature close to boiling point; this condensation reaction is preferably carried out in several stages.
The chief disadvantage of urea-formaldehyde resins is that they cause large quantities of free formaldehyde to be given off. Attempts have been made to reduce the free formaldehyde content by using various methods of manufacture; unfortunately, when the aim is to attain particularly low formaldehyde contents, experience shows that this objective is accompanied both by a decrease in the reactivity and in the stability of the resins and by a deterioration in the mechanical properties of the finished boards. In order to get rid of the free formaldehyde it has also been proposed to employ resins devoid of formaldehyde, particularly resins based on isocyanate solutions. Unfortunately, the problem of the isocyanates is also liable to present toxicological problems.
Another means of limiting the emissions of free formaldehyde consists in adding substances which fix this compound. Among these substances, there may be mentioned, for example, urea, melamine, phenol or else flours with a high protein content. Unfortunately, all the processes using these additives which are capable of fixing formaldehyde when they are added to the urea-formaldehyde resins have awkward secondary effects; in particular they retard the final condensation and consequently the curing of the adhesives.
Among the processes which make use of melamine, it has been proposed, in particular, to add melamine in the final condensation stage at the end of the condensation of urea and of formaldehyde. While a process of this kind makes it possible to obtain resins which can be used to manufacture particle boards exhibiting tolerable free formaldehyde contents combined with acceptable mechanical properties, it leads, on the other hand, to resins exhibiting high gel times. These high gel times mean that the urea-formaldehyde resin containing melamine to which a cure catalyst has been added will take a long time to cure when heated, and this means that, on a production line, the board cure cycle times are long, which reduces the output rates.
There is therefore a need to develop urea-formaldehyde resins containing not more than 10% of melamine and exhibiting reduced gel times.